1. Field of Application
The present invention relates generally to an apparatus for slip forming concrete using a pan which initially begins the concrete forming process of such a machine and to an apparatus for inserting dowel bars for a concrete slip forming machine, and more particularly to such a method and apparatus which inserts dowel bars directly between spaced apart portions of the pan of such machine, instead of utilizing a separate dowel bar insertion device disposed behind the pan, which has been the custom of the prior art.
2. Description of Prior Art
In concrete slip forming machines used for constructing roads and the like, it is customary to form joints therein at predetermined intervals. One of the reasons for these joints is to transfer stresses between adjacent sections of the concrete slabs through the use of dowel bars placed within the slab. Another reason for the joints is to allow for expansion and contraction of the slabs, which occurs during freezing and thawing cycles. The joints are generally perpendicular with respect to the length of the slab and direction of forward movement of the machine. The joints may also extend across the width of the slab at an angle, which is commonplace in present day road construction so that each set of tires of a vehicle does not hit the joint at precisely the same time, thereby lessening the thumping problem that often occurs when the concrete joints are perpendicular to the direction of the movement of the vehicle traveling thereon.
The dowel bars are typically inserted into the concrete at one-third to one-half the depth of the slab down from the top surface of the slab and are generally aligned with the forward movement of the machine. The dowel bars are also usually epoxy coated to prevent rusting of the bars and are usually coated with a film oil to prevent the concrete from bonding to the surface of the bars. After the concrete has set, a saw is used to cut a joint in the concrete above and perpendicular to the dowel bars. The saw-cuts control the shrinkage cracking of the concrete during the final curing stage of the concrete by allowing the concrete to crack more easily along the saw cut joints. The concrete slabs are therefore able to move independently as they expand and contract during temperatures changes but the dowel bars joining the slabs are still able to transfer the shear stresses from slab to slab as motor vehicles pass over the slabs.
The aforementioned concrete slip forming machines have for many years used dowel bar insertion mechanisms to place the dowel bars within the concrete as the slab is being formed, for example like those shown in U.S. Pat. Nos. 4,798,495 and 4,799,820, both by Laeuppi, et al., both of which are incorporated herein by reference.
Conventional dowel bar inserting equipment is attached to the frame of a slip forming machine behind the pan or mold, for example as shown in U.S. Pat. No. 5,190,397 to Bengford, which patent is incorporated herein by reference. By utilizing this prior art technology with the dowel bar insertion mechanism located behind the pan, the concrete slab, which has already been formed, shaped and smoothed by the pan, is disturbed considerably by the insertion of the dowel bars therein. This disruption of the smoothed concrete surface is often referred to in the industry as "scaring" of the surface. The scaring of the surface, due to the insertion of the dowel bars into the formed, slab creates a need for an additional troweling procedure following the dowel bar insertion mechanism to repair the scaring. Such a trowel is shown in U.S. Pat. No. 5,061,115 to Godbersen, et al. Additionally, a vibrating screed or tamper bar must precede the trowel on such a machine in order to consolidate the concrete back around the inserted dowel bar.
The need to have this dowel bar inserting apparatus and accompanying trowel mechanism makes it necessary to lengthen the machine by a considerable amount. This additional length creates many problems such as making the machine more difficult to move and requiring much more time to assemble and disassemble as these machines are moved from one job site to the other. Additionally, on roads that have sharp vertical curves up or down, if the paver is too long, it may not correctly pave the surface thereunder. For example, on a sharp downward vertical curve, the paver may span completely across such a low spot resulting in the paver not being capable of reaching low enough to maintain a uniform slab thickness. Alternatively, on a sharp upward vertical curve, the paver may extend completely across the high spot leaving only a thin layer of concrete. Furthermore, a machine which has a conventional dowel bar inserter thereon and a follow-up trowel mechanism almost always needs to be a four-track machine, which increases the costs of manufacture and use over that of a two-track machine, and also causes the aforementioned problem relating to the difficulty of being able to pave roads which have sharp vertical curves up and down.
U.S. Pat. No. 5,209,602, issued May 11, 1993 to Gary L. Godbersen, which is incorporated herein by reference, solved many of the aforementioned problems. The present invention is an improvement thereto since it has been determined that it is better to form a wide space in the pan than to merely have openings which extend therethrough for inserting dowel bars.
Consequently, there is a need for a method and apparatus for inserting dowel bars on a concrete slip forming machine which will overcome the aforementioned disadvantages of the prior art.